In order to promote a decrease in the weight of automobiles and increase the safety in collisions of automobiles, high tensile strength steel sheets are widely used as a material for members which bear a load at the time of an impact (referred to below as impact-resisting members) which are among the components of automobile bodies. In general, the strength of a steel sheet is influenced by the rate of deformation. As the strain rate of a steel sheet at the time of deformation increases, the deforming stress of the steel sheet increases. A steel sheet having a significantly high tensile strength at the time of a high rate of deformation is suitable as a material for impact-resisting members.
Patent Document 1 discloses a cold-rolled steel sheet with improved impact resistance having a dual-phase structure of ferrite and 10-50% by volume of martensite. This cold-rolled steel sheet is improved in the dynamic deformation properties (the difference between the tensile strength at a high tensile strain rate and the tensile strength at a low tensile strain rate) by reducing the content of solid solution elements dissolved in the ferrite. As a result, it has an increased yield strength when undergoing a high strain rate tensile deformation. Although not specified in Patent Document 1, the tensile strength of a steel sheet having the chemical composition and properties disclosed in Patent Document 1 is thought to be around 590 MPa.
Patent Document 2 discloses a method of manufacturing a high-strength steel sheet which has an ultrafine structure and excellent dynamic deformation properties and which has ferrite grains refined to such an extent that they have a size on the order of nanometers by repeatedly carrying out rolling on a plurality of steel sheets which are stacked. However, because this method requires performing rolling a plurality of times on a plurality of stacked steel sheets, its productivity is extremely poor.
Patent Document 3 discloses a method of manufacturing a cold-rolled annealed sheet having an ultrafine ferritic structure by carrying out cold rolling with an overall rolling reduction of at least 20% and less than 80% on a hot-rolled steel sheet containing at least 90% of a martensitic phase followed by low-temperature annealing at 500-600° C. However, because this method uses a hot-rolled steel sheet having a martensitic phase as a material for working, the material being rolled takes on a high strength and is hardened during cold rolling, and cold rollability markedly worsens, leading to a low productivity.
As disclosed in Non-Patent Document 1, it is known that uniform elongation of a steel sheet markedly decreases as grains are refined.